1. Technical Field
This invention relates to setting an operating frequency in a network. The network could be a wireless telecommunications network such as a cellular radio network.
2. Discussion of Related Art
FIG. 1 shows schematically the configuration of a typical wireless cellular telecommunications network. The network comprises a number of base-stations (BSs) 1, 2, 3 etc. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the area of an associated cell 4, 5, 6. By means of these signals, the base-stations can communicate with a terminal 9 which may be a mobile station (MS) in the associated cell. That terminal itself includes a radio transceiver. Each base station is connected via a base station controller (BSC) 7 to a mobile switching centre (MSC) 8, which is linked in turn to the public telephone network (PSTN) 10. By means of this system a user of the mobile station 9 can establish a telephone call to the public network 10 via the base station in whose cell the mobile station is located. The location of the terminal 9 could be fixed (for example if it is providing radio communications for a fixed building) or the terminal could be moveable (for example if it is a hand portable transceiver or “mobile phone”).
In networks that operate according to the GSM (Global System for Mobile communications) standard base stations must maintain a relative frequency accuracy of 5×10−8 on the air interface between them and mobile stations. One way to achieve this accuracy would be to provide a highly accurate clock at each base station. However, clocks of the required accuracy would generally be too expensive for this approach to be economical.
In commercial networks the normal solution is to implement a single central highly accurate reference clock (11 in FIG. 1) for the network. A clock signal from this clock is then conveyed as a pulse train (illustrated at 12) along the national telephone backbone, and then along the GSM infrastructure (via the MSC and the BSC) to each base station. The central reference clock typically has a relative frequency stability of 10−11 over 24 hours. However, the transmission chain to a base station can be long, and this introduces jitter and wander in the clock signal as received by the base station. The base station typically relies on receiving the signal with an accuracy of 1.5×10−8 at its 2 MBit/s PCM (pulse code modulated) Abis interface. The transcoder inside the base station typically has a 16 MHz clock (divided down to 2 MHz). This is phase locked to the received PCM clock pulses, jitter and wander above 2 Hz is filtered out, and the signal is averaged over a period of approximately 15 minutes. Having been cleaned in this way the 2 MHz clock signal has an improved accuracy and serves as a reference clock for a 26 MHz clock for the base station. All frequencies and timing on the air/radio interface of the base station are ultimately derived from this 26 MHz clock.
This method has a number of drawbacks. First, it relies on there being a continuous stream of pulses to the base station. If the network carrying the pulse fails then the frequency transmission chain from the fixed network to the base station is broken and accurate synchronisation of the radio network is lost. Also, if part of this transmission chain to the base station runs across a non-clocked network then there may be very significant jitter in the pulse train received at the base station. This is a particular concern for systems where the radio network is integrated with a packet-based network such as the internet or an intranet, which is used to carry traffic between the base station and the external telephone network. One example of such a system is the WIO/GIO (Wireless Intranet Office/GSM Intranet Office) system under development by the applicant. In that system it has been proposed that the BSC should be implemented as a distributed unit, with its components being interconnected over an IP (internet protocol) based network such as a company's intranet. IP-based protocols may then be used for all communications, for instance by employing the H.323 protocol for transmission of speech, and signalling. IP networks are not clocked since they operate asynchronously, and accordingly transmission times are highly variable and unpredictable. The components of the proposed WIO/GIO system that are most important in the present context are the A-gateway to the MSC and the IMC (Intranet Mobile Clusters) which each connect to one base station with a PCM or HDSL (High bit-rate Digital Subscriber Line) link, since these would conventionally be expected to be used for transmission of the PCM clock pulses to the base stations. Transmission and network access times are highly variable and unpredictable on IP networks. On a single LAN, transmission times typically are below 10 ms in low traffic situations. In an extensive intranet, transmission times can be higher. Access times are always negligible if the network is not congested. Providing additional cables for carrying a clock signal works against the primary reason for using the intranet: making better use of an existing network.